1. Field of the Invention
The present invention relates to a semiconductor laser driving apparatus used for recording and reproducing data in an optical disc apparatus, and an optical disc apparatus including the same.
2. Description of the Related Art
Recently, an optical disc is expected to be used as a recording medium of video data replacing video tapes, in addition to an external recording medium for a personal computer. Video data encompasses an enormous amount of data. A high recording density is needed to record such an enormous amount of data on a small-diameter optical disc. The following is available as means for achieving a high recording density.
Means 1: introduction of a PWM (pulse width modulation) system, by which information its recorded as the positions of edges of recording marks.
Means 2: introduction of recording correction technology for correcting positions of edges of recording marks so as to reduce a mark edge shift which occurs due to thermal interference between the recording marks at the time of recording.
Means 3: reduction in the level of noise generated in a reproduction system.
For a conventional rewritable optical disc medium, a PPM (pulse position (or phase) modulation) system is generally used, according to which information is recorded as positions of recording marks. According to the PWM system, unlike the PPM system, information is recorded as positions of edges of recording marks. Therefore, one recording mark cat provide two pieces of information. This is advantageous for increasing the recording density. However, it is necessary to precisely control the positions of the edges, which in turn requires precise control of positions of pulses of a recording current for recording the recording marks and also requires precise control of balance of the amount of heat which acts on the recording marks.
FIG. 13 shows recording pulses 133 conventionally used for the PWM system and recording marks 132 which are recorded by the pulses 133 on the optical disc. A recording current 131 includes the pulses 133 for recording the recording marks 132 and a bias power current 136. Each pulse 133 includes a plurality of multi-pulses 134. Each multi-pulse 134 Includes a peak power level 135 and a trough power level 137.
As shown in FIG. 13, each pulse 133 has peak power levels 135 (high level) and trough power levels 137 (low level) appearing alternately. In the case where recording is performed on an optical disc Including a phase shift recording layer, a phase shift phenomenon is utilized to perform recording. More specifically, each mark 132 (i.e., amorphous state) is recorded by the peak power levels 135 of the corresponding pulse 133, and a space (i.e., crystalline state) is recorded by the bias power current 136 between the pulses 133. The trough power levels 137 of each multi-pulse 134 is used to raise the cooling speed of the recording layer.
When the density of the recording marks 132 is increased by the PWM system, each recording mark 132 is shorter than a spot diameter (not shown) of the semiconductor laser. Thermal interference between the recording marks 132 occurs. As a result, the positions of edges of the recording marks 132 deviate from the positions at which the edges should be placed. In order to avoid this, it has been proposed to correct the pulses 133 of the recording current 131 in consideration of the deviation in the positions of edges of the recording marks 132 as described above as means 2.
When the recording marks 132 are shorter than the spot diameter of the semiconductor laser, an amplitude of a reproduction signal generated from the recording marks 132 is restricted by an optical resolution of light emitted from the semiconductor laser and is thus reduced. FIG. 14 is a graph illustrating the relationship between the amplitude C (carrier level) of a reproduction signal and the frequency F of recording marks of different lengths, i.e., a longer recording mark and a shorter recording mark. The shorter recording mark is recorded at a higher density than the longer recording mark. The relationship is obtained by measurement performed on a spectrum analyzer. As shown in FIG. 14. as the length of the recording mark is decreased, the amplitude of the reproduction signal generated from the recording mark is decreased, in proportion to the optical resolution of light emitted from the semiconductor laser. In consideration that it is necessary to obtain a sufficient signal-to-noise ratio oven at a high recording density, reduction of the level of noise included in the reproduction signal is required in correspondence with the reduction in the amplitude of the reproduction signal. The noise included in the reproduction signal from an optical disc includes a semiconductor laser driving current noise NK which is generated in a semiconductor laser driving circuit for driving the semiconductor laser, a semiconductor laser noise NL generated in the semiconductor laser, and a media no,se ND caused by the shape of a groove on the optical disc.
In conventional optical disc apparatuses, the semiconductor laser noise NL occupies a very large part of the noise included in the reproduction signal. In order to improve the quality of high density data recording, it is demanded to provide a semiconductor laser driving apparatus for reducing the semiconductor laser noise NL.
Hereinafter, an exemplary conventional optical semiconductor laser driving apparatus including a semiconductor laser driving apparatus will be described.
FIG. 15 is a schematic diagram illustrating a structure of a conventional optical disc apparatus 1500.
The optical disc apparatus 1300 includes a spindle motor 3 for rotating an optical disc 1, an optical pickup 2 for executing recording of a recording mark on the optical disc 1 or reproduction of the recording mark recorded on the optical disc 1, and a control block 7 for controlling the optical pickup 2 and the motor 3. The control block 7 includes a recording signal processing block 4, a reproduction signal processing block 5, and a central processing block 6. The central processing block 6 includes a laser timing control section 61 and a formatting section 62.
The optical disc apparatus 1500 operates in the following manner.
The optical disc 1 is rotated in a certain direction by the spindle motor 3. The optical pickup 2 receives gate signals A, B, C and D. The gate signals A, B, C and D cause a semiconductor laser (not shown in FIG. 15) in the optical pickup 2 to emit light based on pulses of a recording current. In other words, recording marks are recorded based on the pulses of the recording current. The gate signals A, B, C and D will be described in detail later. The optical pickup 2 also optically detects the recording marks recorded on the optical disc 1, converts the recording marks into electric signals F, G, H, and I, and outputs the electric signals F, G, H and I to the reproduction signal processing block 5.
The central processing block 6 controls the reproduction signal processing block 5 using a control signal J, and controls the recording signal processing block 4 using a control signal K. The central processing block 6 also controls a rotation on speed of the spindle motor 3 using a rotation speed control signal L. The optical disc apparatus 1500 further includes an interface (not shown) for connection with an external device.
FIG. 16 is a schematic diagram illustrating a structure of the optical pickup 2. FIG. 16 mainly shows a portion of the optical pickup 2 for converting an electric signal into an optical signal and converting an optical signal into an electric signal, and portions related thereto.
The optical pickup 2 includes a semiconductor laser 21 for directing light toward the optical disc 1 (FIG. 15) so as to record recording marks on the optical disc 1 based on the recording current or reproduce recording marks recorded on the optical disc 1 based on a reproduction signal. The optical pickup 2 further includes a semiconductor laser driving apparatus 22 for driving the semiconductor laser 21 and a photodetector unit 521 for reproducing the recording marks based on the light from the semiconductor laser 21 which is reflected by the optical disc 1. The photodetector unit 521 includes photodetectors 23 for detecting the reflected light and converting the detected light into a detection signal N, and a head amplifier 24 for generating a reproduction signal based on the detection signal N.
The optical pickup 2 operates as follows.
The semiconductor laser 21 receives a driving current M from the semiconductor laser driving apparatus 22 and converts the driving current M into light.
For recording the recording marks, the semiconductor laser 21 is provided with e recording current which has been modulated into pulses as the driving current M by the semiconductor laser driving apparatus 22.
For generating a reproduction signal for reproducing the recording marks recorded on the optical disc 1, the semiconductor laser 21 is supplied with a reproduction current (DC current) as the driving current M by the semiconductor laser driving apparatus 22. The current K is converted into light and output from the semiconductor laser 21. The light from the semiconductor laser 21 is reflected by the optical disc 1. Depending on whether there is a recording mark or not, the amount, the polarization angle or the phase of the reflected light changes. This change is detected by the photodetector 23 and converted into the detection current N. The detection current N is converted into a voltage by the head amplifier 24 by I-V conversion, and thus becomes a reproduction signal including focus signals F and G and tracking signals H and I. The reproduction signal is supplied to the reproduction signal processing block 5. Then, the reproduction signal processing block 5 executes focus/tracking control. By adding the focus signals F and G and the tracking signals H and I and extracting a high frequency component, pit information corresponding to the recording marks 132 recorded on the optical disc 1 is reproduced.
FIG. 17 is a schematic diagram illustrating a structure of the semiconductor laser driving apparatus 22.
The semiconductor laser driving apparatus 22 includes a recording and reproduction current generation section 518, a high-frequency current generation section 519, and a current driving section 511.
The recording and reproduction current generation section 518 includes current switching blocks 501 through 504, a reproduction power current source 505, a peak power current source 506, a bias power current source 507, and a trough power current source 508, and an addition block 510.
The high frequency current generation section 519 includes a high frequency superposition control section 512, an AC power supply 513 and a capacitor 514. The high frequency current generation section 519 generates a high frequency current including a high frequency component for reducing the level of semiconductor laser noise included in the reproduction signal.
The current switching block 501 switches a reproduction current on or off. The current switching block 502 switches on or off a peak current included in the recording current. The current switching block 503 switches on or off a bias current included in the recording current. The currant switching block 504 switches on or off a trough current included in the recording current.
The reproduction current, the peak current, the bias current and the trough current which are switched on or off by the current switching blocks 501 through 504 are respectively provided by the reproduction power current source 505, the peak power current source 506, the bias power current source 507 and the trough power current source 508. Values of the reproduction current, the peak current, the bias current and the trough current are respectively set by a reproduction power setting signal O, a peak power setting signal P, a bias power setting signal Q and a trough power setting signal R in accordance with predetermined laser powers (reproduction power, peak power, bias power, and trough power).
The gate signals A through D mentioned above are, more specifically, a reproduction power gate signal A input to the current switching block 501, a peak power gate signal B input to the current switching block 502, a bias power gate signal C input to the current switching block 503, and a trough power gate signal D input to the current switching block 504. Whether or not the value settings of the reproduction current, the peak current, the bias current and the trough current is made effective is respectively determined by the gate signals A through D.
The reproduction current, the peak current, the bias current and the trough current generated in this manner are synthesized by the addition block 510 into a recording current having pulses. The recording current is amplified by the current driving section 511 into the driving current M.
The high frequency superposition control block 512 is connected to the addition bloc 510 through the AC power supply 513 and the capacitor 514. For reproduction, the high frequency superposition control block 512 superposes a high frequency current including a high frequency component which is substantially 300 MHz on a reproduction signal for driving the semiconductor laser 21. The high frequency component included in the high frequency current is preferably 300 MHz, which is about 10 times the reproduction frequency band.
The high frequency component, which is substantially 300 MHz, superposed on the reproduction current reduces the semiconductor laser noise NL generated by “mode hopping” and improves the signal-to-noise ratio of the reproduction signal. Whether the superposition is preformed or not (i.e., superposition/non-superposition switching) is controlled by a high frequency superposition gate signal E which is applied to the high frequency superposition control block 512.
The current driving section 511 has a frequency characteristic which enhances a high frequency component included in a high frequency current generated by the high frequency current generation section 519 at the time of reproduction and enhances a high frequency component included in a recording current generated by the recording and reproduction current generation section 518 at the time of recording.
Hereinafter, an operation of the conventional semiconductor laser driving apparatus 22 will be described with reference to FIG. 18.
FIG. 18 it a timing diagram of an optical output from the semiconductor laser 21 (FIG. 16), the reproduction power gate signal A, the peak power gate signal B, the bias power gate signal C. the trough power gate signal D, and the high frequency superposition gate signal E. FIG. 18 also shows a waveform of the optical output (i.e., reproduction/recording current M) from the semiconductor laser 21. In the example shown in FIG. 18. the gate signals A through E are set to be active at a high (H) level.
When the semiconductor laser driving apparatus 22 begins a reproduction operation, the reproduction power gate signal A is placed in an active state (i.e., a high level), and the semiconductor laser 21 starts emitting light based on a reproduction current 1873 (DC signal). Since the high frequency superposition gate signal E is simultaneously placed in an active state, a high frequency current 1875 of 100 MHz or more, for example, in the vicinity of 300 MHz, is superposed on the reproduction current 1873 as the driving signal M of the semiconductor laser 21. Therefore, the semiconductor laser 21 emits light based on a recording pulse current shown in FIG. 18 which is obtained as a result of superposing the high frequency current 1875 on the reproduction current 1873.
For recording, the level of each of the peak power gate signal B, the bias power gate signal C, and the trough power gate signal D changes in accordance with the pattern of the recording marks to be recorded. Based on the peak power gate signal B, the bias power gate signal C, and the trough power gate signal D, a recording current 1674 is corresponding to the pattern of the recording marks to be recorded is generated by the above-described operation of the reproduction power current source 505 (FIG. 17), the peak power current source 506, the bias power current source 507, the trough power current source 508, and the addition block 510. The current driving section 511 enhances the high frequency component included in the generated recording current 1874.
The semiconductor laser 21 emits light having pulses which are substantially the same as those of the recording current 1874, thus recording the recording marks on the optical disc 1 (FIG. 15).
Power values of the reproduction power current source 505, the peak power current source 506, the bias power current source 507 and the trough power current source 508 have the relationship of the reproduction power current source 505<the trough power current source 508<the bias power current source 507<the peak power current source 506. The power values are set in the following manner.
Where currents used for forming the driving current M to be supplied to the semiconductor laser 21 by the reproduction power setting signal O, the peak power setting signal P, the bias power setting signal Q and the trough power setting signal R are respectively currents IO, IP, IQ and IR, the power value of the reproduction power current Source 505 is set so as to correspond to the value of the current IO. The power value of the trough power current source 508 is set so as to correspond to the value of the currents (IO+IR). The power value of the bias power current source 507 is set so as to correspond to the value of the currents (IO+IR+IQ). The power value of the peak power current source 506 is set so as to correspond to the value of the currents (IO+IR+IQ+IP). The power values are set by superposing the currents in this manner.
An optical disc apparatus for recording data on a phase shift recording layer by the PWM system using the above-obtained recording current is described in Nikkei Electronics, No. 7.00 (publication date: Oct. 6, 1997).
The optical disc apparatus 1500 including the semiconductor laser driving apparatus 22 shown in FIG. 17 suffers from the following problems when actually operating.
Problem 1: The shape of the recording marks recorded on the optical disc is not uniform due to dispersion in the composition of the recording layer of the optical disc.
Problem 2: The shape of the recording marks recorded on the optical disc is not uniform due to fluctuations in linear velocity of tracks of the optical disc with respect to the optical pickup occurring during recording.
Problem 3: is the case of high density recording, optimum recording is not realized merely by correcting the positions of the edges of each recording mark. It is also necessary to correct the power value of the recording power for each recording mark.
Problem 4: A high frequency signal superposed on the reproduction signal generates unnecessary radiation of high frequency noise to the outside of the semiconductor laser is driving apparatus.
Problem 5: Provision of a low pass filter for the purpose of reducing the semiconductor laser driving current noise NK results in the waveform of the recording current being less sharp.
The above-mentioned problems will be described below.
<Problem 1>
In the case of an optical disc utilizing a phase shift phenomenon, recording marks are recorded as follows. The temperature of a first portion of the recording layer in which a recording mark is to be recorded is rapidly raised with laser power of more than a certain level, thereby placing the first portion in an amorphous state. The temperature of a second portion of the recording layer in which a recording mark is not to be recorded is gradually raised with laser power of a lower level than that used in the first portion, thereby placing the second portion in a crystalline state. The level of temperature rise which is necessary to realize the amorphous state varies in accordance with parameters including the thermal absorption ratio and thermal diffusion constant.
FIG. 19 shows a shape of a recording mark 132A recorded on a recording layer having a high thermal absorption ratio using the pulses 133 (FIG. 13) of the recording current 131. As shown in FIG. 19, a leading edge 132B of the recording mark 132A is not normally shaped and tapered.
In the case of the PPM system, the information is recorded as the position of each recording mark as described above. Therefore, tapering of the recording mark does not greatly influence the reproduction signal. In the case of the PWM system, by contrast, tapering of the recording mark generates the following inconvenience. The shape of the leading edge 132B is not normal with the positions of the edges being deviated from the intended positions, and thus Jitter is increased at the leading edge 1323. As a result, an error rate of the reproduction signal is raised.
<Problem 2>
When the linear velocity of tracks on the optical disc with respect to the optical pickup is increased due to the fluctuations in the rotation speed of the optical disc, the level of the temperature rise of the recording layer caused by the pulses 133 (FIG. 13) is reduced. As a result, the recording mark 132A shown in FIG. 19 having the tapered leading edge 132B is obtained. Thus, an error rate of the reproduction signal is increased.
<Problem 3>
According to the PWM system, the balance of the amount of heat which acts on the recording marks is precisely controlled by using multi-pulses as described above.
When a 3T mark (T=width of detection window) shown in FIG. 20 is recorded, the following problem occurs. The 3T mark has a pulse width of 3T, which is the shortest possible pulse width realized by 8-16modulation. The pulse width 3T (i.e., the length of the recording mark) is shorter than a diameter DI of a spot of light directed by the semiconductor laser. In an experiment performed by the present inventors, the 3T marks actually recorded were longer than intended. The present inventors found that in order to record a 3T mark properly, it is necessary for a pulse width W2 of a multi-pules 134A for recording the 3T mark to be shorter than a pulse width W1 of a multi-pulse 134 for recording a 4T or longer mark (e.g., a 5T mark shown in FIG. 20).
When the pulse width W2 is shorter, the temperature of the recorded 3T mark itself does not reach a thermal saturation point, As a result, the recorded 3T mark is unstable is terms of shape.
<Problem 4>
It has been reported that a high frequency component (included in a high frequency current) of about 300 MHz to about 450 MHz during a reproduction operation is effective in reducing the semiconductor laser noise NL included in a reproduction signal. A high frequency current including such a high frequency component preferably has the largest possible amplitude. The reason is that as the amplitude of such a high frequency current is larger, the duty of the waveform of light emitted by the semiconductor laser is reduced, namely, the time period in which the semiconductor laser does not emit light is extended. Therefore, interference of the light emitted by the semiconductor laser to an optical disc and the light returning to the semiconductor laser after being reflected by the optical disc can be alleviated. Thus, mode hopping in the semiconductor laser is more unlikely to occur, and the semiconductor laser noise NL included in the reproduction signal is reduced. According to an experiment performed by the present inventors, the amplitude of a high frequency current effective for reducing the semiconductor laser noise NL was 50 mApp at 300 MHz.
However, when a high frequency current of 300 MHz having an amplitude of 50 mApp is transmitted, a loss in the amplitude is generated during the transmission, as a result of which the high frequency current has a smaller amplitude when being input to the semiconductor laser. In order to provide the semiconductor laser with a high frequency current having a larger amplitude, the current value of the high frequency current generated by the high frequency current generation section 519 (FIG. 17) can be increased. However, this increases unnecessary radiation of high frequency noise to the outside of the semiconductor laser driving apparatus, which is a problem with respect to safety standards.
In order to enhance the high frequency component included in the high frequency current at the time of reproduction, the current driving section 511 may be designed to have a frequency characteristic having a frequency peak at 300 MHz. FIG. 21 shows a waveform of a recording current generated when the current driving section 511 has such a frequency characteristic (waveform (b)) in comparison to a waveform of a normal recording current (waveform (a)). The waveform (a) is obtained when the current driving section 511 does not have a frequency characteristic having such a frequency peak. The recording current shown by waveform (b) has excessive overshoot and undershoot respectively at the rising and falling of the pulses. Therefore, it is impossible to provide a recording mark having proper recording characteristics. However, due to restrictions on circuit design or the like, it is very difficult to provide the current driving section 511 with a frequency characteristic which is flat from a low frequency to a high frequency of 300 MHz.
<Problem 5>
Problem 5 occurs when a driving current for driving the semiconductor laser itself includes a noise component (semiconductor laser driving current noise NK), not the semiconductor laser noise NL caused by the semiconductor laser. Since the amplitude of a reproduction signal is reduced as the recording density is increased as described above it is demanded to significantly reduce the level of noise.
When a low pass filter is provided for cutting out noise in a reproduction signal band, the level of noise in the reproduction signal can be reduced in a reproduction operation. FIG. 22 shows a waveform of a recording current which has passed through a low pass filter (waveform (b)) in comparison to a waveform of a normal recording current (waveform (a)). Pulses of the recording current shown by waveform (b) has blunted edges as compared to the edges of the recording current shown by waveform (a). Therefore, the recording sensitivity is lowered.